Clever drinkers have come up with an ingenious way of telling the time while boozing – by turning a beer glass into a sundial.

Friends Jackie Jones and Steve Chapman spent six months perfecting the glass, which when positioned correctly casts a shadow over the time of day. The pair came up with the idea after Steve asked Jackie, a professional sundial maker, to design a beer glass sundial to use at a beer festival he organises. Six months later they launched the SunGlass – and they have since sold more than 400. The clever device works by positioning it so the sun shines through a ring on the back of the glass onto vertical markings showing the months of the year. The height of the shadow then tells the user what time of day it is – and it is accurate to within a few minutes. Like all sundials, the SunGlass uses local solar time, a method of measuring time using the height of the sun, rather than Greenwich Mean Time, which clocks go by. Because of that it doesn’t take into account British Summer Time.

The SunGlass has been calculated to work anywhere in the world which is at a latitude of 51 degrees north, including Banff in Canada, Dresden in Germany and Kazakhstan.